ECG signal are used not only for diagnostic purposes, such as when a trained cardiologist reads the signals but such signals also provide a timing source for synchronizing or triggering other equipment or systems to or by the heart's cycle. Such other equipment or systems may be used for example for imaging (Gamma Cameras, X-ray equipment, MR imagers etc.), diagnostic and therapeutic (e.g. heart pacers) purposes. The most commonly used method of synchronizing with the ECG signal involves feature recognition. The ECG signal comprises cyclical variations, corresponding to the electrical events occuring during variations in the heart's cycles. Certain features of ECG signals have traditionally accepted names, such as P,Q,R,S or T waves. The most prominent feature of the signals the "R" wave is the part usually used for synchromization or triggering purposes. It is generally obtained in skin surface electrodes used to detect the electrical activity of the heart. Another traditionally used variation is called "P" waves and is derived from electrodes placed in the vena cava, or in the esophagus for example.
It has long been recognized that ECG analysis provides more qualitative data when the ECG is made while the heart is under stress such as during exercise testing because most heart attacks occur when the heart is under stress. The ECG trace may give a perfectly "normal" impression when the patient is at rest, and show abnormalities only during exercise.
An ever present problem of obtaining useful signals during exercise ECG is that the movement of the patient during the exercise procedure increases the noise; and therefore, seriously deteriorates the signal-to-noise-ratio (SNR). Also the patient's movement causes muscles to generate electrical signals that compete with the heart's electrical signals. Further, the patient's stepped-up respiration causes cyclical base line wander. Additionally, the patient's perspiration acts to vary the contact resistance between the patient's skin and the sensing electrodes during the test and to thereby vary the detected signals during the test.
The noises, base line wander and varying contact resistance tend to mask the desired signals such as the "R" signal or to provide spurious signals which may be wrongfully interpreted as being a useful signal such as an "R" wave signal. The ECG signals obtained from patients with healthy hearts are harder to mask. Similarly, with a healthy heart it is harder to mistakenly interpret a noise spike as an "R" wave signal because of the regularity of ECG signals obtained from healthy hearts.
On the other hand the ECG signals from people with heart problems are irregular in time, shape and amplitude. Accordingly it is much easier to err when trying to process ECG signals obtained with an unhealthy heart especially when the signals are acquired in the noisy background inherent with exercise ECG.
The problems with noise are aggravated in many ways in systems where the ECG sitnals are analysed and processed by computers and the noise can be often easily mistaken for ECG signals.
Some of the prior art procedure and/or equipment for improving the signal-to-noise-ratio include noise filters. Whereas the noise filters are effective for line frequency caused noises, for example, they are not effective to minimize some other noises such as, for example, signals emanating from other muscles which are in the same frequency range as the heart signals. Other systems, using software, for example, to reduce base line wander take time and cannot be implemented on-line. The medical authorities (such as the A.H.A.) take a dim view of filtering out noise, for fear that a meaningful signal will also be masked out.
Timing systems based on ECG signals work in parallel to the normal recording and analysis, so that filtering is permitted on the timing portion of the ECG signal. Current systems use band pass filters to minimize low frequency base-line wander and high frequency noise. The filtered signal or its derivative is then passed through a "gate" which detects high (absolute) values. As the electrical signals differ from heart to heart, and from time to time in the same heart, and from one electrode position to another at the same time for the same heart, no universal "gate" exists. Rather, most systems use a gate that is determined by the data. The feature of the ECG signal (say, the R-wave) is considered "detected" if a value exceeding a threshold is found. This threshold is generally a certain percent (such as for example 80%) of the last determined peak. This is based on the assumption that the feature being detected is indeed the most prominent feature of the detected signal. Another safety mechanism commonly used is to blank the detector for a certain period (such as 200 msecs) after each detection, it being physiologically impossible for the heart cycle to be that short.
These types of prior art systems, like other systems, have two possible errors: false positives (a detection is declared where it should not be) and false negatives (a feature, say an R-wave, passes without detection). A system is called "sensitive" if it has few false negatives (it detects nearly all the pertinent features), and is called "specific" if it has few false positives (it detects almost only the pertinent features). For a given system, changing the system parameters usually changes both qualities, so that if the sensitivity is raised, the specificity is lowered (it detects more true features, but also more false ones). Alternatively, if the specificity is raised the system's sensitivity is lowered (it detects less errors, but also less true events.
For most purposes, with equipment or systems needing synchronization or triggering with the heart, a sensitivity of 100% is necessary (all true events are detected). Software methods are sometimes subsequently used to reduce the number of false detections, but this is time consuming.
False positives may occur either because of base-line wander, if the base-line strays by more than the said percentage, for example, of the previous peak, or because of high frequency noise or because of an external signal (say, a signal from another muscle), if its amplitude is high enough. False negative may occur either because of base-line wander, if the base-line strays in the opposite direction from the feature, say by 40% the next feature may not reach 80% of the last one. False negatives may also occur because of high frequency noise or an external signal, for example, if a false positive occurred less than the blanking period before the true feature, thereby blanking out the true feature. Noisy events such as an exercise testing are prone to both type of errors.
Accordingly, there is at present a need for systems and procedures to ensure detection of all the occurrences of certain features while reducing the number of false detections. The detection should preferably be done on-line parallel with other uses of the ECG.More particularly, there is a present need for systems and procedures to separate the noise experienced in detecting heart signals during exercise ECG tests from the useful signals.
In the following description, without meaning any loss of generality, "R wave" is referred for short, to mean in general any selected prominent feature which be used for synchronizing or triggering.